Time & Deep Time

Want to study history? In American schools, most intro histories start with homo sapiens, us, and the European variety of us. But Earth's history is far longer. To get a sense of the scale of Earth history, imagine walking back in time, 100 years per step—every pace equal to more than three human generations. A... Continue Reading →

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Gneiss and Schist at a Lake Michigan Beach

Granite is an igneous rock, but over time and under great pressure, granite can become either gneiss or schist, metamorphic rocks characterized by bands or stripes. Such boulders along the Lake Michigan shoreline may have been placed by residents as barriers between yard and beach. Rusty red stains may appear. Gneiss and schist at a... Continue Reading →

Archean Banded Iron Formation

In its youth, our planet was molten, red-hot, and battered by abundant meteorites. Earth was so hostile to life that scientists named this period the Hadean Eon (4.6–4 billion years ago), for Hades, the Greek god of the place called Hell. The earliest known life appears in the geologic record about 3.8 billion years ago... Continue Reading →

Colorful Rocks of the Great Lakes 4

In its earliest youth, our planet was molten, red-hot, and battered by abundant meteorites. Earth was so hostile to life that scientists named this period the Hadean Eon (4.6–4 billion years ago), for Hades, the Greek god of the place known as Hell. Life began to evolve after Earth settled down a bit, in the... Continue Reading →

Colorful Rocks of the Great Lakes 3

Banded Iron Formation (BIF) is a sedimentary rock, but because its extreme age, Archean BIF is highly metamorphosed. It has been compressed, "cooked," and shot through with veins of quartz and granite. The best source of BIF in the United States is north of my location on Lake Michigan. Lots of it was transported here... Continue Reading →

Colorful Rocks of the Great Lakes 2

Archean Eon (4 to 2.5 billion years ago) rocks are frequently deep-water sediments like Banded Iron Formation (BIF). Archean BIF is found in strata no younger than 2.5 billion years, making it one of the oldest rock-types on Earth. Northern Minnesota contains bountiful Archean BIF formations that have been mined for iron ore. Glaciers brought... Continue Reading →

Colorful Rocks of the Great Lakes 1

How lucky am I? Some of the oldest rocks on Earth lie north of my location on Lake Michigan. A half-dozen glacial waves carried rocks from the Precambrian exposures in the northern Great Lakes to my beach in southeastern Wisconsin. Characterized by stripes of red iron-oxide particles, these rocks are called Banded Iron Formation (BIF).... Continue Reading →

Olivine, a Semiprecious Stone

The dark-green spots on these stones are the mineral olivine, or peridot. Olivine is special. Geophysicists believe olivine was the first mineral to form on a young Earth. Finding olivine-bearing rock where I live is extra-special. Olivine is igneous in origin. At the south end of Lake Michigan, bedrock is very deep sedimentary. No volcanoes... Continue Reading →

Basalt Ventifact

A ventifact is a rock that has been carved by wind-driven sand or ice crystals. This basalt rock with the eerie, ghostlike pattern is a ventifact or the result of water drip-drip-dripping over time. Copyright, Disclaimer, Glossary, Bibliography

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